graphene for hydrogen evolution reaction

Accepted Manuscript Title: Insight into the excellent catalytic activity of (CoMo)S2 /graphene for hydrogen evolution reaction Authors: Li Xin Chen, Zhi Wen Chen, Ying Zhang, Chun Cheng Yang, Qing Jiang PII: DOI: Article Number:

S0926-3373(19)30758-1 https://doi.org/10.1016/j.apcatb.2019.118012 118012

Reference:

APCATB 118012

To appear in:

Applied Catalysis B: Environmental

Received date: Revised date: Accepted date:

25 April 2019 20 July 2019 25 July 2019

Please cite this article as: Chen LX, Chen ZW, Zhang Y, Yang CC, Jiang Q, Insight into the excellent catalytic activity of (CoMo)S2 /graphene for hydrogen evolution reaction, Applied Catalysis B: Environmental (2019), https://doi.org/10.1016/j.apcatb.2019.118012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Insight into the excellent catalytic activity of (CoMo)S2/graphene for hydrogen evolution reaction

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Li Xin Chen,‡ Zhi Wen Chen,‡ Ying Zhang, Chun Cheng Yang,* Qing Jiang*

Key Laboratory of Automobile Materials (Jilin University), Ministry of Education,

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and School of Materials Science and Engineering, Jilin University, Changchun

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130022, China

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These authors contributed equally to this work.

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‡

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Corresponding authors. Tel.: +86-431-85095371; Fax: +86-431-85095876; E-mails: [email protected] (C. C. Yang); [email protected] (Q. Jiang).

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Graphical abstract

In this work, a strategy is proposed to activate the inert surfaces of MoS 2 and to boost electrons transport through alloying with non-noble metal Co and compositing with graphene based on DFT simulations. The as-designed (CoMo)S2/graphene composite

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shows superior catalytic activity for hydrogen evolution reaction with the onset potential of 28 mV and a low overpotential of 100 mV at a current density of 10 mA cm-2, outperforming most of MoS2-based catalysts.

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Highlights An HER catalyst of (CoMo)S2 was designed on the basis of DFT simulations.

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The hybrid activates inert surfaces of MoS2 and boosts the conductivity.

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(CoMo)S2 shows excellent HER catalytic performance.

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The mechanisms behind improved HER performance are revealed.

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

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ABSTRACT

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Electrochemical water splitting has become a potential pathway for sustainable

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hydrogen production, where high-efficiency low-cost catalysts are highly desirable. MoS2 is a promising candidate to substitute effective but expensive Pt-based catalysts

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for hydrogen evolution reaction. In this work, the computer simulation results reveal

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that alloying with Co and compositing with graphene could significantly improve the electrocatalytic performance of MoS2 thanks to the activation of MoS2 inert surfaces and enhanced electron transport. Based on this finding, we designed and synthesized a

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catalyst of (CoMo)S2/graphene, which exhibits excellent performance with low onset potential (28 mV), overpotential (100 mV) for driving a current density of 10 mA cm-2, and Tafel slope (60.8 mV dec-1), superior to most of MoS2-based electrocatalysts reported in open literatures. This study supplies important information for

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fundamental understanding of high-efficiency non-noble electrocatalysts toward applications in energy-conversion devices.

Keywords: Hydrogen evolution reaction; Catalysts design; Density functional theory

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simulations; MoS2; Graphene

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1. Introduction Energy crisis and climate change derived from fossil fuel combustion have become major concerns. The exploitation of clean and renewable energy sources is thus imperative. H2 has gained increased interests because of the highest energy

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density of 142 MJ kg-1 [1-3]. The electrochemical water splitting is an economical and effective approach to produce H2 [4,5], where highly active catalysts are needed in

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order to achieve low overpotentials and fast kinetics [6-8]. The current hydrogen

evolution reaction (HER) electrocatalysts are Pt and Pt-based materials, which are

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scarce and expensive, severely limiting their applications [9-11]. Thus, it is highly

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desirable to explore non-noble HER electrocatalysts with high efficiency [12-15].

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MoS2, as a two-dimensional (2D) material with intriguing physical and chemical

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properties, has gained tremendous attentions for HER during the past few years

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[16-22]. In general, the active centers of MoS2 are considered as coordinatively unsaturated edge sites due to their appropriate hydrogen adsorption free energy, while

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the in-plane domains of MoS2 are inert [23,24]. To further boost the HER activity of

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MoS2, two strategies were frequently utilized through increasing exposed edges and improving intrinsic activity of active sites per unit area [25,26]. On the basis of these two strategies, various approaches have been proposed. Chatti et al. prepared a

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composite of vertically-oriented MoS2 nanosheets on graphene support by using microwave synthesis, which shows outstanding electrocatalytic performance due to high-concentration edge sites and improved electrical conductivity [15]. Jaramillo et al. enhanced the catalytic activity of MoS2 via preferentially exposed edges [27]. Ye et

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al. introduced numerous edge sites through O2 plasma and H2 treatments on monolayer MoS2, leading to high catalytic activity [28]. Yang et al. fabricated edge-oriented MoS2 film with more active sites, delivering superb catalytic performance [29]. Compared with edge sites, the activated inert basal planes of MoS2

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showed greater potential for catalyzing HER through single metal atom doping, or surface S-vacancy formation [16,30-32].

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Recently, Bao’s group introduced single-atom Pt into the basal plane of MoS2 nanosheets (Pt-MoS2) [16]. A significant improvement of HER activity on Pt-MoS2

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with high stability was achieved in the acidic medium. Moreover, the in-plane

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domains of monolayer MoS2 were also activated via creating S vacancies [30]. The

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corresponding catalytic performance could be regulated by adjusting strain and

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vacancy concentration. However, the doping of noble Pt and complex preparation

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process of S vacancies increase the electrocatalysts cost, thus limiting their applications. Moreover, the low defect concentrations from noble metal doping and

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S-vacancy creating impeded HER activity of MoS2 [16,30]. On the contrary, alloying

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with non-noble Co into MoS2 is an economical method to activate abundant inert surfaces [33]. It has also been demonstrated that non-noble metals (e.g., Co, Ni) doping could improve the catalytic activity of MoS2 via adjusting its electronic

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structure [24,34].

Based on the aforementioned considerations, our strategy strives to activate in-plane domains of MoS2 through Co alloying to form (CoMo)S2. Moreover, our recent work has demonstrated that reduced graphite oxide (RGO) is beneficial for

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high electrical conductivity and suitable hydrogen adsorption free energy [35]. Thus, in this work, we designed a hybrid HER catalyst of (CoMo)S2/RGO under the guidance of density functional theory (DFT) simulations. As expected, the composite shows an outstanding catalytic performance with the onset potential of 28 mV and a

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small overpotential of 100 mV at a current density of 10 mA cm-2 because of the synergistic effects between (CoMo)S2 and RGO. The low cost and outstanding

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catalytic performance make (CoMo)S2/RGO a potential candidate to replace Pt-based catalysts toward HER.

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2. Catalyst design aided by DFT simulations

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2.1 Simulation method

[36,37].

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generalized

gradient

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design

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In this work, the spin-polarized DFT in the DMol3 code is used for the catalyst approximation

(GGA)

with

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Perdew-Burke-Ernzerhof (PBE) describes exchange-correlation effects [38]. The relativistic effect is considered by a DFT semi-core pseudo potentials (DSPPs) core

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treatment [39]. This means that core electrons are described by a single effective

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potential. Moreover, the basis set is double numerical plus polarization (DNP) with fine quality for the orbital cutoff. The geometrically optimized structures are obtained until the maximum displacement, maximum energy gradient, and energy change

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become less than 5.010-3 Å, 2.010-3 Ha/Å, and 1.010-5 Ha, respectively. The corresponding k-point sampling of the Brillouin zone and the smearing parameter are set as 441 grid and 5.010-3 Ha, respectively. The computational hydrogen electrode (CHE) model is used for calculating free

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energies [40]. The adsorption free energy of hydrogen (GH*) is given by GH* = EH* + ZPE - TS + GU, where EH*, ZPE, T, S and GU are the adsorption energy of hydrogen, zero point energy, absolute temperature, entropy change, and free energy contribution from the electrode potential (U), respectively. Furthermore, EH*

the adsorbed system, the isolate system, and 1/2 H2 (gas), respectively.

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2.2 DFT-aided catalyst design

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is determined by EH* = EH* – Ecat –EH, where EH*, Ecat and EH are total energies of

The conductivity of electrocatalysts plays a vital role for HER, which requires

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fast electron transfer in the catalytic processes [30,41]. Unfortunately, the pristine

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monolayer MoS2 owns a large direct band gap of 1.80 eV [42], indicating poor

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electrical conductivity. To conquer this drawback, two strategies were used: (1)

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alloying with Co to form the impurity levels; and (2) compositing with a conductive

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graphene.

Firstly, we vindicate the accuracy of our simulation method by calculating the

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band gap of the pristine monolayer MoS2, which is 1.74 eV at K-point (see Fig. 1a

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and 1b), consisting with the reported value of 1.80 eV [42]. Fig. 1c and 1d display the geometric optimization structure and band structure of (CoMo)S2, respectively. After alloying with Co, the Fermi level of (CoMo)S2 moves up 1.20 eV in comparison to

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that of pristine monolayer MoS2, implying n-type doping of MoS2. A large number of electrons transfer from Co to adjacent S of MoS2 and the band gap decreases from 1.74 eV of MoS2 to 0.28 eV of (CoMo)S2 due to the formation of impurity levels (see Fig. 1d), which is beneficial for electrocatalysis [34]. To further increase the

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electronic conductivity, graphene is introduced to form the (CoMo)S2/graphene heterointerface and the corresponding geometric optimization structure and band structure are shown in Fig. 1e and 1f, respectively. It can be found that (CoMo)S2/graphene endows a similar conductive behavior to graphene. In addition,

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the Fermi level shifts down 0.20 eV compared with that of (CoMo)S2 due to the electron transfer from (CoMo)S2 to graphene. The binding energies of MoS2/graphene

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and (CoMo)S2/graphene calculated are -2.23 and -2.30 eV, respectively, which indicate that the heterointerface structure becomes more stable after Co alloying.

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Furthermore, the hydrogen adsorption free energy GH* was calculated to

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evaluate the HER performance. As shown in Fig. 1g, the HER pathway can be

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depicted through an initial state of H+ + e-, an intermediate state of adsorbed H (H*),

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and a final state of 1/2 the H2 product [43]. In the volcano plot of HER, the GH*

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value on the volcanic vent is about 0 eV, indicating the optimal HER activity [23]. The inert plane of MoS2 owns a GH* value of 1.93 eV, which implies an

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energetically unfavourable adsorption process of hydrogen, resulting in a sluggish

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Volmer reaction. As shown in Fig. 1c, the inert S (site “1”) is activated by the nearest neighbor Co due to the electron transfer from Co to S. The calculated GH* is -0.18 eV, implying an appreciable HER activity. The corresponding adsorption structure is

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shown in Fig. S1. More intriguing is that the (CoMo)S2/graphene endows a near zero GH* of -0.12 eV, which is much closer to that of Pt (GH* = -0.09 eV). Moreover, the GH* values of other sites (sites “2”, “3” and “4” in Fig. 1c and 1e) were also calculated and listed in Table S1. Only the site “2” has a catalytic activity for HER

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while other sites are inert. As a result, the (CoMo)S2/graphene should possess excellent HER performance with high electrical conductivity and appropriate GH* value through rational design based on DFT simulations. 3. Experimental section

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3.1 Materials Cobalt dichloride (CoCl2·6H2O) and ammonia water (NH3·H2O) were bought

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from Sinopharm Chemical Reagent Co., Ltd. Ammonium tetrathiomolybdate

[(NH4)2MoS4], hydrazine hydrate (N2H4·H2O) and graphite flake were bought from

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Sigma Aldrich. The above chemicals are of analytical purity and applied directly.

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3.2 Catalyst preparation

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Graphene oxide (GO) was synthesized according to a modified Hummers’

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method [44], which was described in detail elsewhere [45]. 0.48 ml GO (roughly

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corresponding to a molar ratio of MoS2/RGO = 5) was then dispersed in ultrapure water to form the dispersion liquid (0.5 mg ml-1). For the synthesis of (CoMo)S2

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precursor, 1 mmol (NH4)2MoS4 and 1 mmol CoCl2·6H2O were dispersed in 20 ml

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ultrapure water, respectively. Then, the CoCl2 solution was added into the (NH4)2MoS4 solution drop by drop under stirring for 30 min. Subsequently, the (CoMo)S2 precursor were incorporated with the GO dispersion. After sonicating for

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30 min, 4.72 l N2H4H2O and 75.6 l NH3·H2O were added into the mixture. Then, the mixture was put into an oil bath (95 C) for 1 h, reducing GO into RGO. The resulting suspension was filtered, washed and dried at 60 C overnight. After that, the fabricated composite was annealed at 500 C for 5 h under N2 atmosphere. The final

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product was treated with HCl (3 M), washed by using ultrapure water and ethanol, and dried at 60 C. For comparison, (CoMo)S2 was prepared under the same process without the co-reduction treatment of GO. Moreover, to optimize the RGO content in (CoMo)S2/RGO, two other samples were prepared using 0.24 ml and 2.40 ml GO

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(roughly corresponding to molar ratios of MoS2/RGO = 10 and = 1, respectively) as the precursor in this work.

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3.3 Catalyst characterization

X-ray diffraction (XRD) was performed on a D/max2500pc diffractometer with

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Cu Kα radiation (λ = 0.15406 nm). Field-emission scanning electron microscope

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(FESEM, JSM-6700F, JEOL, 15 keV) and transmission electron microscope (TEM,

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JEM-2100F, JEOL, 200 keV) were used to characterize samples’ surface morphology

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and microstructure. X-ray photoelectron spectroscopy (XPS) was carried on an

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ESCALAB 250 spectrometer with a monochromatic Al-K (1486.6 eV) source. Raman spectrum was recorded on a micro-Raman spectrometer (Renishaw) using a

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laser source with 532-nm excitation wavelength. The nitrogen adsorption and

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desorption measurements were performed on a Micromeritics ASAP 2020 analyzer to determine the specific area and pore size distribution of samples.

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3.4 Electrochemical measurements Electrochemical

measurements

were

performed

on

an

Ivium-n-Stat

electrochemical workstation in a standard three-electrode system at room temperature. The working electrode is a glassy carbon rotating disk electrode (RDE, Pine Research Instrumentation) with a diameter of 5 mm, which is covered by a thin catalyst film. A

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graphite rod and a saturated calomel electrode (Hg/Hg2Cl2, SCE) are used as the counter electrode and reference electrode, respectively. Typically, catalyst ink was prepared as follows. 3 mg catalyst powders were dispersed in 0.5 ml mixture of water-isopropanol solution (4:1, v/v) and 50 l 5 wt% Nafion solution. After that, the

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above mixture was sonicated for 30 min to generate a uniform ink. Then, 30 l as-obtained catalyst ink was pipetted using a micro-pipettor and loaded onto the

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freshly polished glassy carbon electrode (GCE). After naturally drying at room temperature, the catalyst film was finally obtained. Similar preparation methods have

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been reported in open literatures [9,15,33]. Here, isopropanol can disperse the solid

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mixture, which is beneficial for the formation of uniform film to cover the whole

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GCE after the drying of the catalyst ink. Nafion was used as a reinforcing layer for

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robust films adhered to the electrode surface, resulting in a superior durability of the

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electrode during electrochemical measurements [46]. Linear sweep voltammetry (LSV) measurement was conducted on RDE in 0.5 M sulfuric acid (H2SO4) at a

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rotation rate of 2025 rpm and a scan rate of 5 mV·s-1. Note that the disk electrode’s

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planner surface with the catalyst has direct contact with the electrolyte solution. When the electrode is rotating, the solution runs from the bulk to the electrode surface, and then flows away along the direction parallel to the disk surface [47]. Electrochemical

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impedance spectroscopy (EIS) was performed at an overpotential () of 0.15 V (vs. RHE) from 100 K Hz to 0.1 Hz [33]. Stability tests were conducted using continuous cyclic voltammetry (CV) from -0.15 to 0.1 V (vs. RHE) with a sweep rate of 100 mV s-1. The time dependence of the current density is measured at a static overpotential of

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-0.08 V (vs. RHE) for 20000 s. All potential data were calibrated with respect to the RHE via E(RHE) = E(SCE) + 0.273 V. Note that the electrolyte solution was purified with N2 for 30 min to remove oxygen completely before electrochemical tests. The hydrogen production efficiency experiments were conducted in 0.5 M H2SO4 at a

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potential of -0.3 V (vs. RHE) for 220 min. Thereinto, Ar gas was delivered into the electrolyte with a rate of 30 ml/min (at ambient pressure and room temperature) and

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routed directly into the gas sampling loop of a gas chromatograph (GC-2014). The composition analysis of gas phase products was performed every 20 min using GC.

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The gas sample was routed through a packed MoleSieve 5A column and a packed

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4. Results and discussion

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Moreover, helium was used as a carrier.

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Rt-Q-BOND column before passing a pulsed discharge detector for H2 quantification.

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Based on the DFT-aided catalyst design, a (CoMo)S2/RGO composite was fabiricated by chemical synthesis, annealing and acid treatment, which is

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schematically shown in Fig. 2. As comparative samples, bare MoS2 and (CoMo)S2

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were also fabricated. To study the influence of different RGO contents on the catalytic activity of (CoMo)S2/RGO for HER, three samples were synthesized using different amounts of GO (0.24, 0.48, and 2.40 ml). It is found that 0.48 ml is the optimal

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amount, which will be discussed below. Thus, the aftermentioned (CoMo)S2/RGO composite denotes the sample fabricated by adding 0.48 ml GO unless otherwise specified. Fig. 3 shows the microstructure and morphology characterizations of the

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(CoMo)S2/RGO composite. From the XRD patterns (see Fig. 3a), the diffraction peak at around 26.4° for (CoMo)S2/RGO can be indexed to RGO with graphitic structure, while it is absent for (CoMo)S2. All rest diffraction peaks correspond well to CoMoS3.13 (PDF Card No. 16-0439) [48], indicating that (CoMo)S2 maintains the

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layered structure of MoS2 after Co alloying. Note that some S atoms are missing due to alloying with low-valent Co during the preparation process. Moreover, the (002)

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peak (at around 14.2°) intensity of (CoMo)S2/RGO is weaker than that of (CoMo)S2, suggesting the prevention of the (CoMo)S2 stacking via introducting RGO. This

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would provide more active sites for HER [31]. The structure of (CoMo)S2/RGO was

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further investigated by the Raman spectroscopy. As shown in Fig. 3b, the two

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dominant peaks at 377 and 402 cm-1 are ascribed to the E12g and A1g modes of MoS2,

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respectively. The A1g mode shows much higher intensity than that of the E12g mode,

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indicating edge-terminated structure of (CoMo)S2/RGO. Such a structure is beneficial for the HER catalytic activity [33,49,50]. Moreover, RGO is also found in the Raman

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spectrum (typical D band at 1359 cm-1 and G band at 1581 cm-1, respectively), which

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is consistent with the XRD pattern. The high intensity ratio of ID/IG (~1.29) indicates a large amount of defects and disorders in RGO [42,51,52]. As a result, there are strong interactions between (CoMo)S2 and RGO, which not only improve the stability

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(CoMo)S2/RGO but also accelerate electron transport in HER. Fig. 3c presents N2 adsorption/desorption isotherms of (CoMo)S2/RGO. The specific surface area measured is 69.9 m2 g-1 based on the Brunauer-Emmer-Teller (BET) model. From the inset of Fig. 3c, the pore size is about 3~25 nm according to the

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Barrett-Joyner-Halenda (BJH) method. The large specific surface area and broad mesopore size distribution originate from the disorder assembly of MoS2 nanoflakes and graphene sheets with different sizes [53], which could provide more active sites for HER and accelerate the H+ and H2 diffusion during the HER process.

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Fig. 3d shows an SEM image of (CoMo)S2/RGO, where the (CoMo)S2 nanoparticles (average diameter of ~70 nm) are formed by stacking (CoMo)S2

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nanosheets. On the contrary, the aggregation is observed (see Fig. S2) for bare (CoMo)S2 synthesized by the same experimental process without RGO owing to high

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surface energy. This suggests that the presence of RGO in the composite can inhibit

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the agglomeration of (CoMo)S2 nanoparticles [42]. Fig. 3e presents a TEM image of

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(CoMo)S2/RGO, where the (CoMo)S2 nanoparticles are uniformly distributed on

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RGO due to the interaction between (CoMo)S2 and RGO. Thus, more active sites

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could be exposed for the composite. Also, the introduction of RGO can enhance the electrical conductivity of (CoMo)S2/RGO. Fig. 3f shows an HRTEM image of

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(CoMo)S2/RGO. The inter-planar spacing of 0.624 nm corresponds to the (002) plane

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of (CoMo)S2, which indicates that the introduction of Co atoms does not change the layer structure of MoS2. The scanning TEM image and energy dispersive X-ray spectrum (EDX) mappings of (CoMo)S2/RGO are shown in Fig. 3g-k, demonstrating

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homogeneous distribution of Mo, S, C and Co in (CoMo)S2/RGO. The quantitative element concentration analysis has been performed for (CoMo)S2/RGO via EDX measurements. The atomic ratio is 3.2:5.4:15.3 for Co:Mo:S. Thus, the ratio of S/(CoMo) is ~1.8, very close to the stoichiometric ratio (= 2) of (CoMo)S2. The small

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difference (~0.2) may be attributed to influences of some intrinsic factors of EDX, such as fluorescence yields, X-ray adsorption, detector collection angle, detector efficiency etc. [54,55]. The surface electronic states of (CoMo)S2/RGO were characterized by XPS.

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From the survey spectrum of (CoMo)S2/RGO (see Fig. 4a), Mo, S, Co, C and O elements are clearly indentified. Fig. 4b-f plots high-resolution XPS spectra of Mo 3d,

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S 2p, C 1s, Co 2p and O 1s, respectively. As shown in Fig. 4b, six fitted peaks exist in the Mo 3d spectrum, where the peak located at 226.5 eV can be assigned to S 2s of

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MoS2. The peaks located at 229.2 eV and 232.3 eV are typical characteristic peaks

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(Mo 3d5/2 and 3d3/2, respectively) of MoS2, while the relatively higher binding energy

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peaks of Mo 3d5/2 (229.5 eV) and Mo 3d3/2 (232.8 eV) indicate the existence of Mo5+

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ions. The peak at 235.9 eV can be attributed to MoO3, which is possibly generated

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during the catalyst preparation and exposure to air [56,57]. The sulfur species are identified from S 2p spectrum (see Fig. 4c). The peaks of S 2p3/2 (162.0 eV) and S

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2p1/2 (163.2 eV) are assigned to the S2- of MoS2. Additionally, the S 2p3/2 (162.7 eV)

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and 2p1/2 (164.1 eV) show the presence of bridging S22-. The existence of S with a higher binding energy indicates potential HER active sites [58-60]. For C 1s XPS spectrum (see Fig. 4d), the weak peak intensities of C-O and C=O bond demonstrate

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that the oxygen-containing groups in RGO have been removed successfully [51,61]. The Co 2p spectrum is fitted with four doublets (see Fig. 4e). The first one at 779.2 eV and 794.1 eV are attributed to 2p3/2 and 2p1/2 of the CoMoS phase. The second one at 780.5 eV and 796.9 eV are Co2+ 2p3/2 and Co2+ 2p1/2, respectively. The third and

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fourth belong to the satellite peaks [33,62]. For the O 1s spectrum (see Fig. 4f), the peaks of C-O, C=O and Mo-O bonds are located at 532.4, 531.7, and 529.9 eV, respectively, which indicate that the O atoms come from the residual GO and partial oxidation of the sample during the preparation process.

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To investigate the HER performance in an acidic electrolyte (0.5 M H2SO4), electrochemical measurements were performed using an RDE apparatus in a standard

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three-electrode system. Fig. 5a and 5b show the LSV and corresponding Tafel plots, respectively. The commercial Pt/C, bulk MoS2 and (CoMo)S2 were also measured for

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comparisons.The efffets of bare GCE and RGO on the electrochemical performance

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are negligible due to their little activity for HER (see Fig. S3). As shown in Fig. 5a,

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the best electrocatalyst Pt/C exhibits the smallest onset potential (0 mV) and the

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lowest overpotential (38 mV) at a current density of 10 mA cm-2, which is

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subsequently used as a reference to other catalysts [63,64]. The bulk MoS2 presents almost no catalytic activity because of its slow electron transport and inert basal plane.

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The (CoMo)S2 catalyst shows a lower onset potential of 128 mV compared with bulk

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MoS2 thanks to the activation of inert planes of MoS2 by alloying with Co. From Table 1, the (CoMo)S2/RGO composite exhibits a lower overpotential of 100 mV for driving a current density of 10 mA cm-2 among MoS2-based electrocatalysts reported

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in open literatures apart from P-MoS2@hierarchical carbon microflower (86 mV) and Pd-MoS2 (78 mV). Moreover, (CoMo)S2/RGO possesses the onset potential of 28 mV, which is much lower than those of other electrocatalysts as listed in Table 1 (except for MoS2/Ti3C2-Mxene@C). The composition effect on HER catalytic performance of

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(CoMo)S2/RGO was investigated by considering different amounts of the precursor of GO. As shown in Fig. S4, the (CoMo)S2/RGO hybrid synthesized by adding 0.48 ml GO exhibits the lowest overpotential of 100 mV at 10 mA cm-2, relative to other two samples (144 mV for 2.40 ml GO and 200 mV for 0.24 ml GO). Thus, 0.48 ml is the

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optimum adding amount, which ensures a balance between the electrical conductivity and active sites number. Fewer GO could not provide sufficient electrical conductivity

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while excess GO would cover some active sites of the catalyst.

To further substantiate the elevated HER efficiency of (CoMo)S2/RGO catalyst,

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the corresponding Tafel slopes were measured, as shown in Fig. 5b. Pt/C shows a

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Tafel slope of ~31.0 mV dec-1, which is consistent with the previously reported values.

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This indicates that our electrochemical measurements data are reliable [16,60]. The

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Tafel slope of (CoMo)S2 is 145.6 mV dec-1, which is better than that of bulk MoS2

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(197.9 mV dec-1). Meanwhile, the (CoMo)S2/RGO displays a much smaller slope (60.8 mV dec-1) compared with (CoMo)S2 and bulk MoS2, demonstrating higher

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catalytic activity.

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The hydrogen evolution mechanism can be elucidated by the Tafel slope. In acidic media, three possible reaction steps are involved in HER process, which are H3O+ + e-  H* + H2O (Volmer reaction), H* + H3O+ + e-  H2 + H2O (Heyrovsky

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reaction), and H* + H*  H2 (Tafel reaction). The Tafel slope value of 120, 40, or 30 mV dec-1 indicates that the Volmer, Heyrovsky, or Tafel reaction is the rate-determining step, respectively. Here, the Tafel slope of (CoMo)S2/RGO catalyst is 60.8 mV dec-1, which indicates a Heyrovsky-dominated Volmer-Heyrovsky

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mechanism [35]. Note that RGO contributes significantly to the enhanced electrical conductivity and thus high HER catalytic performance of (CoMo)S2/RGO. EIS tests were carried out to evaluate the electrode kinetics for HER (see Fig. 5c). It is found that the Pt/C electrode has the smallest charge-transfer resistance (~2 ) among the

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samples, indicating the fastest electron transport. The Nyquist plots reveal that (CoMo)S2/RGO shows a remarkable decrease in charge-transfer resistance (Rct)

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compared with bulk MoS2 and (CoMo)S2. This is caused by the improvement of the

electrical conductivity through introducing Co and RGO, consistent with the DFT

U

simulation results as illustrated in Fig. 1.

N

The electrochemical surface areas of the catalysts were evaluated by the

A

electrochemical double layer capacitances (Cdl) using CV measurements. The

M

potential range (0.45~0.55 V vs. RHE) is first determined from CV with a

ED

non-Faradaic current response. Then, the current density (j) is measured in the above potential range at multiple scan rates of 10~50 mV s-1 (see Fig. S5-S7). Note that j can

PT

be expressed by j = νCDL, where ν and CDL denote the scan rate and electrochemical

CC E

double-layer capacitance, respectively. Therefore, a j-ν plot yields a straight line, where the slope is equal to CDL [65]. The Cdl value of (CoMo)S2/RGO is 11.4 mF cm-2, much higher than those of (CoMo)S2 (8.9 mF cm-2) and bulk MoS2 (0.7 mF cm-2), as

A

shown in Fig. 5d. Moreover, the CDL is also calculated by the EIS measurements. The corresponding CDL values of (CoMo)S2/RGO, (CoMo)S2 and bulk MoS2 are 10.2, 7.7, and 0.5 mF cm-2, respectively, which are close to the corresponding CDL values (11.4, 8.9, and 0.7 mF cm-2) determined by CV. This indicates that (CoMo)S2/RGO

18

possesses the most active sites through alloying with Co and compositing with RGO, and thus endows higher HER activity than those of bulk MoS2 and (CoMo)S2. Another significant criterion for an advanced electrocatalyst is excellent electrochemical stability. Fig. 5e plots the current-time curve of (CoMo)S2/RGO at a

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potential of -0.08 V. The current density decreases slightly after a scanning period of 20000 s, which might due to the consumption of H+ or the accumulation of H2

SC R

bubbles on the electrode surface [60,66]. The inset in Fig. 5e shows an SEM image of

(CoMo)S2/RGO after the 20000-s durability test. It is clear that the (CoMo)S2/RGO

U

hybrid maintained its original structure, indicating a superior cyclic stability.

N

Moreover, we compared the LSV curves of (CoMo)S2/RGO at the initial cycle and

A

after 5000 cycles of CV scanning (see Fig. S8). There is a tiny decay of potential (8

M

mV) for HER on (CoMo)S2/RGO catalyst after 5000 CV cycles, also demonstrating

ED

the superior durability of (CoMo)S2/RGO.

To further substantiate the hydrogen generation efficiency of (CoMo)S2/RGO,

PT

the actual H2 production was measured by gas chromatography and the corresponding

CC E

results are shown in Fig. 5f. The detected H2 amount is consistent with the theoretical value, corresponding to a high Faradic efficiency of about 98%. This confirms high electrocatalytic efficiency of (CoMo)S2/RGO as an HER electrode in producing

A

high-purity H2.

4. Conclusions In conclusion, we have developed a facile synthetic strategy to directly grow nanostructured (CoMo)S2 on RGO as a high efficiency electrocatalyst for HER. The

19

incorporated non-noble Co atoms in (CoMo)S2 nanostructures not only form substantial desirable active sites, but also improve the electrical conductivity. The intrinsic activity and electrical conductivity are further enhanced by compositing with RGO. The optimized (CoMo)S2/RGO hybrid exhibits excellent HER catalytic

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performance, outerporming most of reported MoS2-based electrocatalysts. Both theoretical and experimental results in this work indicate that the (CoMo)S2/RGO

SC R

composite is a promising HER catalyst with high activity, low price and superior

U

long-term stability.

A

N

Acknowledgements

M

This project is financially supported by National Natural Science Foundation of China

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(Nos. 51671092 and 51631004), Program for JLU Science and Technology Innovative Research Team (No. 2017TD-09) and the Fundamental Research Funds for the

A

CC E

PT

Central Universities.

20

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32

Table 1 Comparisons of HER performances among MoS2-based electrocatalysts.

222

152

61 63 57 40 57 58 58 51 70

[71] [72] [73] [74] [75] [76] [77] [78] [79]

86

42

[80]

78

62

[81]

N A

64 130 30

[70]

220 186 150 110

75 110

PT CC E A

33

This work [18] [24] [25] [35] [51] [67] [68] [69]

62.7

U

120 88

Reference

IP T

75 85 30 20 104 100

Tafel slope (mV dec-1) 60.8 51 74 56.9 71.3 67.4 45 76.1 64

SC R

Overpotential (mV) at 10 mA cm-2 100 155 156 171 205 110 135 240 188

ED

(CoMo)S2/RGO Ni-Co-MoS2 Co-mesoporous MoS2 foam CoMoS3 hollow prism NPNi-MoS2/RGO MoS2/graphene MoS2/Ti3C2-Mxene@C Porous MoO2/MoS2 MoS2-rGO/Mo MoS2/multiwalled carbon nanotube 1T/2H MoS2 MoS2/N-Graphdiyne MoS2(1-x)Px MoS2/N-carbon nanotube NF-MoS2 NiS2/MoS2 MoS2-MoP/C Zn-MoS2 MoS2/Ti3C2Tx P-MoS2@hierarchical carbon microflower Pd-MoS2

Onset potential (mV) 28 125

M

Electrocatalyst

235 102

Figure Caption

Fig. 1. DFT simulation results. The geometrically optimized structures and band structures of MoS2 [(a) and (b)], (CoMo)S2 [(c) and (d)] and (CoMo)S2/graphene [(e)

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and (f)]; (g) describes the free energy diagram of hydrogen evolution at “standard” conditions corresponding to 1 bar of H2 and pH = 0 at 300 K. The yellow, azure, cyan

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and grey balls represent S, Mo, Co and C atoms. The numbers in (c) and (e) denote various adsorption sites of H atom. The dotted lines in (b), (d), and (f) indicate the

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Fermi level.

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Fig. 2. Schematic illustration for the synthetic route of the (CoMo)S2/RGO composite.

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Fig. 3. Microstructure and morphology characterizations. (a) XRD patterns of

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(CoMo)S2 and (CoMo)S2/RGO; (b) Raman spectrum and (c) N2 adsorption/desorption

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isotherms and pore size distribution (the inset) of (CoMo)S2/RGO; (d-f) SEM, TEM and HRTEM images of (CoMo)S2/RGO; (g-k) scanning TEM image and the

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corresponding EDX mappings of (CoMo)S2/RGO.

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Fig. 4. X-ray photoelectron spectroscopy spectra. (a) XPS survey spectrum of (CoMo)S2/RGO; (b)-(f) High-resolution XPS spectra of Mo 3d, S 2p, C 1s, Co 2p and O 1s of (CoMo)S2/RGO, respectively.

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Fig. 5. Electrochemical performances. (a) Polarization curves of Pt/C, bulk MoS2, (CoMo)S2 and (CoMo)S2/RGO; (b) Tafel curves of Pt/C, bulk MoS2, (CoMo)S2 and (CoMo)S2/RGO; (c) Nyquist plots for Pt/C (see the inset for an enlarged plot), bulk MoS2, (CoMo)S2 and (CoMo)S2/RGO [the applied potential is 0.15 V (vs. RHE) from

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100 K Hz to 0.1 Hz]; (d) The capacitive currents at 0.5 V as a function of scan rate for bulk MoS2, (CoMo)S2 and (CoMo)S2/RGO; (e) Time-dependent current density curve of (CoMo)S2/RGO under a static overpotential of -0.08 V vs. RHE for 20000 s [the inset is an SEM image of (CoMo)S2/RGO after a 20000 s durability test]; (f) The

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production rate and Faradic efficiency of (CoMo)S2/RGO for hydrogen generation.

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Fig. 1.

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Fig. 5.

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